Semiconductor lasers may be used in a variety of industrial and scientific applications, such as optical communications. Optical communications applications, for example, may employ lasers that emit light at a particular lasing wavelength (e.g., 1.31 μm or 1.55 μm) suitable for transmission through optical fibers. Semiconductor lasers may be desirable over other types of lasers because they have a relatively small volume and consume a relatively small amount of power.
Lasers generally include a laser cavity defined by mirrors or reflectors and an optical gain medium between the reflectors in the laser cavity. When pumped with pumping energy (e.g., an electrical current), the gain medium amplifies electromagnetic waves (e.g., light) in the cavity by stimulated emission, thereby providing optical gain and generating a laser light output. In semiconductor lasers, a semiconductor active layer or region serves as the gain medium and reflectors provide optical feedback for laser oscillation within the active region. In Fabry-Perot lasers, for example, a set of mirrors or cleaved facets bound the active region to provide the optical feedback. In other semiconductor lasers, such as distributed feedback (DFB) lasers and distributed Bragg reflector (DBR) lasers, one or more diffraction gratings (e.g., Bragg gratings) may be used to provide reflectance. In a DFB laser, for example, a distributed reflector (e.g., a diffraction grating or Bragg grating) along the active region provides the optical feedback and may be used to restrict oscillation to a single mode.
Fiber optic communication systems may require a high performance light source capable of generating single-mode, narrow spectral linewidth emission in the 1.3-1.56 μm wavelength range. Some of the existing semiconductor lasers (e.g., InGaAsP DFB lasers) fail to provide stable single-mode operation that is insensitive to ambient temperature change (uncooled operation) and insensitive to external optical feedback (isolator-less operation) and/or fail to provide high single-mode yield and high output power. Complex-coupled DFB lasers have been developed that provide advantages such as high single-mode yield, less sensitivity to external optical feedback, high modulation bandwidth and reduced wavelength chirp. Multiple quantum well (MQW) DFB lasers with etched quantum wells, for example, may provide high single-mode stability.
Complex-coupled DFB lasers generally provide both index coupling and gain coupling. Certain complex-coupled DFB lasers will predominantly lase on the longer wavelength side of the Bragg stop band (i.e., the long Bragg mode). In complex-coupled DFB lasers with periodically etched MQWs, however, there are still fundamental problems such as variations of the complex coupling coefficient due to variations of grating etching depth, laser performance variations due to random variations of facet grating phase, and variations in lasing wavelength due to the ratio of index to gain coupling.